Aftermarket seats..what type do U have?
I've got '99 dodge avenger seats in my '73 duster.
I used the factory front outside hole for reference ( to make the seats the same distance from the door sills)
Then bolted some threaded stock through the factory holes on the avenger seats
Then set the seat hieight by moving the seat up/down on the treaded rod.
tacked the rod in place on the floor.
Then made some brackets that look like this that fit over the threaded rod.
welded to the floor, the bottom edge was cut to match the floor of the car where ever it lined up.
All in all a real pain in the but.. might not do it that way again
******KEEP IN MIND THE TRANS TUNNEL IS NOT IN THE DEAD CENTER OF THE CAR**** I learned the hard way...
I'll get some pics tonight.
In the mean time here are the seats I used.
And look at
http://www.70duster440.com/ in the repairs and mods section for the seat height.. mine are about the same.